The coating of thermoplastic polymer substrates with moisture resistant barrier coating compositions to provide impermeability to gases such as oxygen, and liquids, is known [See, e.g., U.S. Pat. No. 3,282,729]. Alkali metal polysilicates have long been known as protective coatings that modify the permeability or surface characteristics of polymeric films and other articles [See, e.g., U.S. Pat. Nos. 1,007,482; 1,424,425; 1,949,914; 3,102,038; 3,130,061; 3,180,747; 3,492,137; 3,522,066; 3,533,816 and 3,706,603].
Lithium (Li), potassium (K), and sodium (Na) polysilicate are used in coatings for a variety of surfaces. For example, Hecht and Iler, Canadian Patent No. 993,738, describe a gas and liquid-impermeable coating for polymeric substrates comprising lithium polysilicate having a mole ratio of SiO.sub.2 to Li.sub.2 O of between about 1.6 and 4.6.
Although alkali metal polysilicate barrier coatings do provide excellent vapor, gas and aroma barrier properties to coated articles, the authors have found that some metal polysilicate barrier coatings display reduced oxygen barrier performance at relative humidities greater than about 60 percent. Additionally, polymeric films coated with said alkali metal polysilicate barrier coatings exhibit reduced barrier performance when subjected to repeated flexing.
Additionally, polymeric articles are known to become hazy after certain polysilicate coatings have been incorporated. The tendency of sodium polysilicate coatings to effloresce, i.e., to become covered by powdery crystalline material as a result of atmospheric exposure, has been documented [Weldes and Lange, Ind. Eng. Chem., 61(4):28-44 (1969)]. This characteristic has similarly been observed for lithium polysilicate coatings by the present inventors. In contrast, pure potassium polysilicate coatings do not effloresce, but suffer severe loss of barrier performance above 50 percent relative humidity. Pure lithium polysilicate coatings, on the other hand, exhibit little to no loss of barrier performance over the same relative humidity range.
Vermiculite, a layered silicate, has also been employed as an ingredient in coatings of various types. For example, U.S. Pat. No. 5,183,704 refers to so-called 2:1 layered silicates, where the silicates are selected from mica and vermiculite. W. J. Ward et al, J. Membrane Science, 55:173-180 (1991) suggests vermiculite may provide the basis for a variety of improved coatings for films. It is stated that vermiculite could be used in a sprayed-on high barrier coating for low barrier plastics. U.S. Pat. No. 5,244,729 refers to a polymeric film having a coating layer which comprises vermiculite platelets. See, also, U.S. Pat. No. 4,101,050 and European patent applications No. 92305329.2, published Dec. 16, 1992 as EP No. 518,647 and 92305330.0, published Dec. 16, 1992 as EP No. 518,646.
Although layered silicates, including vermiculite, are known to form stable aqueous suspensions, these suspensions are also known to become destabilized in the presence of dissolved metal cations, such as those present in alkali metal polysilicates. Such destabilization results in flocculation of the layered silicates and reduces their efficacy in vapor, gas and aroma barrier applications.
There remains a need in the art for barrier coatings which overcome these deficiencies in prior art coatings.